1. Field of the Invention
The present invention generally relates to the art of fluid compression. More specifically, the present invention relates to methods for controlling the operation of an air compression system having a plurality of rotary screw compressors.
2. Brief Description of the Prior Art
Compression systems having a plurality of individual compressors are generally sequenced so as to improve the overall efficiency of the compression system and minimize premature wearing of the individual compressors. Sequencing has traditionally been accomplished using external devices or controllers. The earliest sequencing devices were electromechanical devices that controlled compressors with a series of staggered pressure bands.
More recent sequencing devices include host controllers with electronic microprocessors for loading and unloading compressors over a single pressure band. These electronic sequencers are also capable of automatically changing the sequencing order of the compressors. Generally, the host controller receives a pressure reading from a supply line pressure sensor and compares the actual pressure reading to a set point pressure range or pressure bandwidth, to determine whether it is necessary to load, unload and/or modulate any of the compressors in the system.
In such conventional compressor systems, one of the compressors is designated the lead compressor for the system and one or more of the compressors are designated the lag compressors for the system. During operation of the conventional compressor system, the supply line pressure fluctuates primarily because of changes in the demand for compressed fluid. As a result, from time to time it is necessary to modulate one or more of the compressors in order to maintain the required system pressure.
FIG. 1 shows a prior art compression system including an external device for sequencing operation of two or more compressors. The compression system 10 includes a host controller 12, a lead compressor 14, a lag compressor 16, and a supply line pressure sensor 18 that is flow connected to compressed fluid supply line 20. In the compression system shown in FIG. 1, the demand for compressed air requires that the lead compressor 14 be continuously operated in a fully loaded state, and the lag compressor 16 is modulated between unloaded and fully loaded operation.
During operation of the compression system 10, the host controller 12 decides when to modulate the lag compressor 16 based upon the pressure level sensed by the supply line pressure sensor 18. The host controller compares the actual sensed supply line pressure with the predetermined set point pressure range stored in the memory of the host controller. When the supply line pressure is outside the acceptable supply line pressure range and is greater than the acceptable set point supply line pressure, the host controller sends a signal to the lag compressor 16 and unloads the compressor until the actual supply line pressure is within the acceptable supply line pressure range. The lead compressor 14 remains fully loaded at all times.
When the actual supply line pressure is less than the acceptable supply line pressure, the host controller 12 transmits a load signal to the lag compressor 16 to fully load the lag compressor, and the lag compressor 16 runs fully loaded until the actual supply line pressure is within the acceptable operating range. When the lag compressor is fully loaded, the lead compressor remains fully loaded. The host controller 12 continuously cycles the compressors between loaded and unloaded modes during operation of the multiple compressor system 10, thereby minimizing wear of the compressors.
Adding a host controller or external device for controlling operation of a plurality of compressors adds significant cost and complexity to an air compression system. Thus, it is apparent that it would be advantageous to provide a method for controlling the operation of a compression system that sequences air compressors over a single pressure band without using an expensive host controller or external device. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.